Answer this question truthfully
### Question: What are the causes of Aphthous ulcer ?
### Response: Aphthous ulcers may be linked to problems with the body's immune (defense) system. The sores may occur after a mouth injury due to dental work, aggressive tooth cleaning, or biting the tongue or cheek. They can also be triggered by emotional stress, dietary deficiencies (especially iron, folic acid, or vitamin B-12), menstrual periods, hormonal changes, food allergies, and similar situations. They occur most commonly with viral infections. In some cases, the cause can not be identified.